Infrared transmitters include a light-emitting diode (hereinafter, referred to as an “LED”) that emits an infrared ray, and uses a light modulation method in which a luminance of the LED is changed according to a signal to be transmitted.
Because a luminance of an LED changes according to a magnitude of a driving current flowing in the LED, the light modulation method is implemented by converting a transmission signal to an electrical current and using the electrical current as the driving current. Here, without a bias current equivalent to, or higher than 50% of the maximum amplitude level of the transmission signal, applied to the LED, a change in the electrical current into which the transmission signal is converted is saturated to hamper the appropriate modulation.
A relationship between a bias current for an LED and a maximum amplitude level of a transmitting signal is described with reference to drawings. In FIGS. 6 and 7, the longitudinal axis represents a driving current for an LED corresponding to an amplitude level of a transmitting signal, while the horizontal axis represents time.
FIG. 6 depicts a relationship between a driving current and a bias current value for an LED in an infrared transmitter that includes two channels (Ach and Bch) and transmits a signal of only a single channel at a time. As shown in FIG. 6, in the infrared transmitter that transmits a signal of only a single channel at a time, I1 equivalent to, or higher than 50% of a driving current maximum value corresponding to a transmission signal of a single channel is set as the bias current value. Thus, even in an infrared transmitter including a plurality of channels, if a signal of a single channel is transmitted at a time, the driving current thereof is free of saturation by setting a value equivalent to or higher than 50% of the maximum amplitude of the signal of the single channel as the bias current value.
FIG. 7 depicts a relationship between a LED driving current and a bias current in an infrared transmitter capable of transmitting signals of the two channels (Ach and Bch) at a time.
As shown in FIG. 7, in transmitting signals of two channels at a time, the transmission signals are superimposed to make amplitude larger and, accordingly, the amplitude of the driving current becomes larger than that in the case of the single channel transmission. Therefore, a bias-current value I2 equivalent to, or higher than 50% of the maximum value of the driving current becomes larger. The infrared transmitter capable of transmitting signals of two channels at a time has the bias current value I2 set as described above even upon single channel transmission. Therefore, a larger current compared with that in the case with I1 (FIG. 6) is consumed, which leads to an unnecessary use of a driving source of the transmitter.
A light modulation method that improves frequency characteristic by changing a driving current is proposed for the infrared transmitter having the problem as described above (see, for example, Patent Document 1).
An infrared transmitter disclosed in Patent Document 1 employs a light modulation method in which a driving current for an LED is changed to keep an output level of a transmitting signal constant and improve frequency characteristics. Still, the infrared transmitter cannot change a bias current according to an output level of a transmission signal.
[Patent Document 1] Japanese Patent Application Publication No. S62-176225